The machines for tearing and moving materials such as earth and rocks, including excavators, loaders and the like, are usually provided with one or more shovels or buckets attached to a mechanical arm. The bucket is provided with a blade or bevelled lip on a front edge thereof intended to engage and penetrate the mass of earth and rocks. To prevent an excessive wear of the lip and to help penetrate the earth, it is common to assemble wear elements associated to the lip and projecting from the front thereof. Such a wear assembly comprises teeth or point members to tear the material up, and tooth-holders or adapters, attached to the lip or the bucket, that join the teeth to the lip.
Said wear elements cannot avoid being also subjected to wear and to large stresses that can deteriorate them, specially the teeth, so that they must be frequently replaced. Also, depending on the work the machine is intended for, it may be desirable to change the type or the shape of the teeth to improve their qualities.
Besides, the type of work in the earthmoving field may vary from light construction to normal construction, heavy construction and mining, and a specific work usually determines the size of the machine and the type of wear assembly, because the mechanical requirements are different.
For example, in construction works the machines are smaller than in mining works and the loads and stresses are bigger in mining works than in public works. With light loads, a two-part wear assembly is mostly used, namely, an adapter attached to the lip, e.g. welded (though another attachment is of course not ruled out), and a tooth coupled to said adapter. Usually, the adapter is configured as a male part and the tooth is configured as a female part, but in some cases the opposite is true and the assembly is then called “inverse system”.
In mining works, where the loads to resist are bigger, it is common to use three-part wear assemblies comprising a point member (tooth), an intermediate member and a cast or weld-on member (adapter). Said three-part assemblies are used when the weight of the assembly is very high, so that increasing the number of elements facilitates their handling. A three-part wear assembly is also useful because the ratio of usable wear material is increased. Usually, the intermediate member is provided with a male part at its front region and a female part at its back region, the point member is configured as a female part and the adapter is configured as a male part. But other dispositions are possible, for examples in inverse systems.
The coupling between the different parts is maintained by means of a retaining element, usually a pin, which can be vertical or lateral.
The service life of the coupling is limited due to:                plastic deformation of the material due to the reactions for counteracting the exerted forces;        fatigue: it is calculated that a tooth with a normal duration performs more than 50,000 work cycles; as a result, the coupling must be designed to prevent the defects occurring because of fatigue phenomena, such as cracks or other defects;        wear, it being necessary to distinguish two types of wear:                    1. Outer wear of the parts due to the flow of the material.            2. Inner wear due to the fine materials that are introduced between the two elements (tooth-tooth bar), whereby an abrasive effect is produced with the movements between the two elements which gradually wears them out.                        
The fitting or attachment of the wear assembly is formed by the contact surfaces of the male and female parts, where the loads are transmitted from the point of the wear element to the lip of the bucket through the adapter element.
When the machine is working, the point of the tooth engages the terrain in different directions, which causes stresses on the tooth that are transferred from the point thereof to the contact surfaces that contact the male part, i.e. they go from the point to the adapter (or to the intermediate element in case of a three-part assembly) and from the adapter to the bucket, so that the stresses are unloaded from the assembly and also from the pin.
In traditional wear assemblies, the housing or cavity in the female part and the complementary nose of the male part are wedge-shaped. Upon applying a force on the tooth, this shape generates reaction forces on the adapter that tend to separate these two elements, or even to break the tooth, and to stress the pin, which can lead to a very dangerous situation because a fallen-out pin can cause the tooth to fall out too, which can be very harmful because a tooth collected as earth can break any subsequent machinery (for example a crusher). Moreover, the loads are transmitted in all directions and tend to deteriorate the wear assembly, particularly the contact or bearing surfaces between the wear assembly elements, and also the pin.
The document WO2007/097984 discloses a wear assembly for securing a wear member to excavating equipment that includes a base having a nose and a wear member having a socket. The nose and socket are each provided with one or more complementary stabilizing surfaces in central portions thereof. But it is found that said surfaces are not bearing enough to stabilize the wear assembly when large loads act in different directions at the same time.